---
title: Custom Renderer Plugins
slug: /custom-renderer-plugins
section: Graphics
---

Excalibur knows how to draw many types graphics to the screen by default comes with those pre-installed into the [[ExcaliburGraphicsContext]]. However, you may have a unique requirement to provide custom WebGL commands into excalibur, this can be done with a custom renderer plugin.

:::warning

This is an advanced API it is recommended you use built in graphics unless you are comfortable with building WebGL geometry and shaders.

:::

## Registering a renderer plugin

Registering a renderer with the graphics context will allow you to call it's draw method during your game.

```typescript
const game = new ex.Engine({...});

game.start().then(() => {
    // register
    game.graphicsContext.register(new MyCustomRenderer());
});

// call from a graphics callback or event
const actor = new ex.Actor({...});
actor.graphics.onPostDraw = (graphicsContext) => {
    graphicsContext.draw<MyCustomRenderer>('myrenderer', ...);
}
```

## Writing a custom render plugin

In order to build a custom renderer extend the [[RendererPlugin]] interface

```typescript
export class MyCustomRenderer extends ex.RendererPlugin {
  /**
   * Unique type name for this renderer plugin
   */
  readonly type: string = 'myrenderer';

  /**
   * Render priority tie breaker when drawings are at the same z index
   * @warning Not yet used by excalibur
   */
  priority: number = 0;

  /**
   * Initialize your renderer
   *
   * @param gl
   * @param context
   */
  initialize(gl: WebGLRenderingContext, context: ExcaliburGraphicsContextWebGL): void {
    // initialize and compile shader
  }

  /**
   * Issue a draw command to draw something to the screen
   * @param args
   */
  draw(some: ex.Vector, args: ex.Color): void {
    // update internal state with draw command with the args
  }

  /**
   * @returns if there are any pending draws in the renderer
   */
  hasPendingDraws(): boolean {
    // if there are any un-flushed drawings
    return false;
  }

  /**
   * Flush any pending graphics draws to the screen
   */
  flush(): void {
    // flush all pending draws to the screen
  }
}
```

For example this is the [[PointRenderer]] implementation

```typescript
export class PointRenderer implements RendererPlugin {
  public readonly type = 'ex.point';
  public priority: number = 0;
  private _shader: Shader;
  private _maxPoints: number = 10922;
  private _buffer: VertexBuffer;
  private _layout: VertexLayout;
  private _gl: WebGLRenderingContext;
  private _context: ExcaliburGraphicsContextWebGL;
  private _pointCount: number = 0;
  private _vertexIndex: number = 0;
  initialize(gl: WebGLRenderingContext, context: ExcaliburGraphicsContextWebGL): void {
    this._gl = gl;
    this._context = context;
    this._shader = new Shader({
			graphicsContext: context,
      vertexSource: pointVertexSource,
      fragmentSource: pointFragmentSource
    });
    this._shader.compile();
    this._shader.use();
    this._shader.setUniformMatrix('u_matrix', this._context.ortho);
    this._buffer = new VertexBuffer({
      size: 7 * this._maxPoints,
      type: 'dynamic'
    });

    this._layout = new VertexLayout({
      shader: this._shader,
      vertexBuffer: this._buffer,
      attributes: [
        ['a_position', 2],
        ['a_color', 4],
        ['a_size', 1]
      ]
    });
  }

  draw(point: Vector, color: Color, size: number): void {
    // Force a render if the batch is full
    if (this._isFull()) {
      this.flush();
    }

    this._pointCount++;

    const transform = this._context.getTransform();
    const opacity = this._context.opacity;

    const finalPoint = transform.multv(point);

    const vertexBuffer = this._buffer.bufferData;
    vertexBuffer[this._vertexIndex++] = finalPoint.x;
    vertexBuffer[this._vertexIndex++] = finalPoint.y;
    vertexBuffer[this._vertexIndex++] = color.r / 255;
    vertexBuffer[this._vertexIndex++] = color.g / 255;
    vertexBuffer[this._vertexIndex++] = color.b / 255;
    vertexBuffer[this._vertexIndex++] = color.a * opacity;
    vertexBuffer[this._vertexIndex++] = size * Math.max(transform.getScaleX(), transform.getScaleY());
  }

  private _isFull() {
    if (this._pointCount >= this._maxPoints) {
      return true;
    }
    return false;
  }

  hasPendingDraws(): boolean {
    return this._pointCount !== 0;
  }

  flush(): void {
    // nothing to draw early exit
    if (this._pointCount === 0) {
      return;
    }

    const gl = this._gl;
    this._shader.use();
    this._layout.use(true);

    this._shader.setUniformMatrix('u_matrix', this._context.ortho);

    gl.drawArrays(gl.POINTS, 0, this._pointCount);

    GraphicsDiagnostics.DrawnImagesCount += this._pointCount;
    GraphicsDiagnostics.DrawCallCount++;

    this._pointCount = 0;
    this._vertexIndex = 0;
  }
}
```
